The invention relates to an impact tool, in particular an impact wrench.
Specifically, but not exclusively, the invention may be used to lock a bolt on a threaded hub of a wheel, for example to change rapidly the wheels of a car during a race.
In particular, the invention refers to an impact tool in which: a rotating mass (hammer), that acts as a flywheel for storing mechanical energy, is rotated by a motor (in generally of the pneumatic type); a rotating shaft is fixed to an anvil rotated by the hammer by means of a series of impacts (generally one impact for each rotation); the connection mechanism between hammer and anvil comprises a clutch that, after each impact, leaves the hammer free again to rotate and can be driven, for example, by a cam system. In use, an external device (for example a screw coupling member) is removably connected to the rotating shaft, generally by interposing a mechanical adapter.
More in particular the invention refers to an impact tool having a torque sensor that is operationally associated with the output shaft. Such a sensor is known, for example, from the patent publication US 2007/0103104 A1.
The use of a pneumatic impact wrench for fitting and removing wheels from a motor vehicle is known, in which the tightness of the (normally alloy) bolt that locks the wheel on the (normally hardened and tempered steel) hub must be safe and reliable, also when the stress transmitted by the wheels to the hub is high, as occurs in a motor vehicle competing in a car race.
It is thus desirable that the screwdriver rotates fast but also ensures appropriate tightness, with a known torque, so as not to damage the bolt, to ensure the maintenance of the coupling between bolt and hub in all conditions, for example during a competition.
It is therefore desirable to stop the screwdriver at the most appropriate moment, neither too soon or too late, such that the tightening works locks the wheel, but without damaging the screw coupling.
Patent publication US 2004/0182587 A1, for example, teaches measuring the torque on the output shaft of a pneumatic impact tool, using contactless sensors of magnetic type and stopping the tool when a threshold value of the detected torque is detected.